Sure you can buy a bunch of unit... but you cannot deploy a lot of them, unless you already have a lot of territory thus a lot of unit already buildt.
I dont think this is a real problem in actual play.
About fraction and rounding up. I thing the good answer is 3x2.5= 7.5 rounded up to 8, this is usually how it work with ADG games.
If the world war artefact is on the progress track:
- all empires taking a production action halve the total
cost of new units they build and can place any
number of units in any area;